The present invention relates to a copper-containing conductor composition, and more particularly to a composition for providing a copper conductor which is formed into conductor patterns or electrodes on, mainly, ceramic substrates. The composition of the present invention can give excellent solderability (wettability by solder), adhesive strength, migration resistance and solder leach resistance to the conductor.
Thick film conductor compositions are composed of, generally, a conductive metal, an inorganic oxide and a glass powder as an inorganic bonding agent, which are in finely divided form and are dispersed in an organic vehicle. The conductive metal is ordinarily gold, silver, palladium, platinum, or mixtures and alloys thereof. However, these metals are noble metals, and they are expensive and have large fluctuations in price. When the noble metals other than gold are used as the conductive metal of the conductor composition, the obtained conductor has many troubles on its properties, for instance, high migration, poor solder leach resistance, high resistivity, and the like.
Accordingly, it has been strongly expected to substitute base metals, particularly copper metal, which are cheap and have small fluctuation in price, for the noble metals.
As to copper conductor compositions, hitherto, there have been utilized as the conductive metal ingredient spherical copper powders prepared in a wet process, a dry process, a combination thereof, and the like. However, commercially available copper powders have an oxide which contains at least 0.2% by weight oxygen as a copper oxide on the surface. That is, it can be considered that the copper powder is naturally oxidized during preparation process thereof or the powder is previously oxidized from the viewpoint of stability of copper. Since copper is very easily oxidized in air, even if the copper powder is subjected to antioxidation treatment, it is oxidized during the treatment to produce the oxide having at least 0.2% by weight oxygen.
The present inventors prepared various copper conductor compositions from the above-mentioned conventional copper powders. As a result, it has been found that the obtained copper conductor, in any compositions, could not satisfy all of properties required as conductor (conductor properties) such as solderability, adhesive strength, matching property to resistance, variation by preparation lots, due to at least 0.2% by weight oxygen in the oxide on the surface of the copper powder. On the other hand, when a copper powder put on the market is washed with an acid such as hydrochloric acid or nitric acid in order to remove an oxide from the surface, it can be considered that though the oxide layer on the surface can be removed therefrom immediately after washing, since the treated copper powder is remarkably easily oxidized during drying or storage, the amount of the oxide is consequently increased compared to the case that the copper is not treated.
An object of the present invention is to provide a copper conductor composition capable of providing a conductor satisfying all of the conductor properties.
This and other objects of the present invention will become apparent from the description hereinafter.